Twin-lens reflex camera

The twin-lens reflex camera is like the single-lens reflex camera in that both have a mirror that reflects an image of the scene upwards. But its mirror does not move; it is fixed in place behind a lens used only for viewing. The camera’s other lens is used to expose the film. The lenses, which are usually stacked one on top of the other, are mechanically coupled so that both are focused at the same time. The lenses must be of exactly equal focus and must focus together. Some models have interchangeable lenses, however most do not. For those that do, both lenses have to be changed at once.

The image reflected by the mirror is displayed on a ground glass viewing screen at the top of the camera, a flat surface that the photographer looks at from above and uses to compose the image. The viewing screen typically has crossed hairlines running through it to aid the photographer in composing. It can be viewed while holding the camera just below the chin, at waist level or even when laid on a table or on the ground. This makes it a particularly convenient camera for photographing flowers, small wildlife, babies or pets. The camera’s viewing screen can even be viewed sideways or upside-down and overhead, with the photographer’s arms stretched out, which can be very handy when shooting a subject at the center of a crowd or when shooting over an obstruction like a fence.

Composing with the twin-lens reflex takes a little getting-used-to because the image on the camera’s viewing screen is a mirror-image – that is, it appears reversed left to right.

Having said that, the twin-lens reflex camera was for years the standard roll-film camera of the professional, and its viewing screen permits remarkably-accurate focusing.

Because the twin-lens reflex camera has a mirror that does not move, its construction is simplified compared with a single-lens reflex camera. There are less moving parts to break down and the camera is inherently fairly rugged. It also has the advantage that the image is seen before, during and after exposure.

It is also a relatively-large camera, in the medium-format range, and loads with 120 or 220-size roll film. (This is why you may sometimes hear it being referred to as a "roll film" camera.) Its size and its top-down viewscreen make it less convenient than most 35mm cameras for candid photography. The twin-lens reflex shares the viewfinder camera’s problem of parallax error because the lens you look through is not the lens that takes the picture, and the slightly-separated perspectives make close-up pictures tricky to frame without cutting off a portion of the subject. Some top-quality twin-lens reflex cameras, however, have built-in automatic parallax correction.

The single-lens reflex camera has pretty much taken over the market that used to be filled by the twin-lens reflex camera because of the SLR's many advantages, and there are few twin-lens reflex cameras available today. However, many owners of twin-lens reflex cameras use them regularly for their picture-taking and rely them upon for superior photography.